The present invention relates generally to magnetic data storage and retrieval systems. More particularly, the present invention relates to a transducing head having a writer grounded to a substrate.
Hard disc drives (HDDs) generally include a transducing head that reads data from and writes data to a magnetic storage medium, such as a disc having a number of concentric data tracks that store data in the form of localized magnetic fields, or bits. The transducing head includes a reader or read head. Several layers typically form the reader, including a top electrode, a bottom electrode and a magnetoresistive (MR) read sensor positioned between the top and bottom electrodes. The electrodes may also function as shields, where the shields ensure that the read sensor only reads that information stored directly beneath it on the magnetic disc.
As the read sensor is positioned above the localized magnetic fields of the rotating magnetic disc, a sense current passed through the MR read sensor allows detection of a time-dependent magnetic field modulation of a magnetization of the read sensor. Different types of MR read sensors are known, including current perpendicular to plane (CPP) readers and current in plane (CIP) readers. A CPP MR read sensor can be of a number of giant magnetoresistive (GMR) read sensor types, including, but not limited to, a tunneling giant magnetoresistive (TGMR) element or a spin valve (SV) element.
GMR read sensors have a series of alternating magnetic and nonmagnetic layers. The resistance of GMR read sensors varies as a function of the spin-dependent transmission of the conduction electrons between the magnetic layers separated by the nonmagnetic layer and the accompanying spin-dependent scattering, which takes place at the interface of the magnetic and nonmagnetic layers and within the magnetic layers.
GMR read sensors using two layers of ferromagnetic material separated by a layer of nonmagnetic electrically-conductive material are generally referred to as SV read sensors. The layers of a SV read sensor include a nonmagnetic spacer layer positioned between a ferromagnetic pinned layer and a ferromagnetic free layer. A magnetization of the pinned layer is fixed in a predetermined direction, typically normal to an air bearing surface (ABS) of the SV read sensor, while a magnetization of the free layer rotates freely in response to an external magnetic field. An antiferromagnetic material is typically exchange coupled to the pinned layer to fix the magnetization of the pinned layer in a predetermined direction, although other means of fixing the magnetization of the pinned layer are available.
GMR read sensors using two layers of ferromagnetic material separated by a layer of nonmagnetic electrically-insulating material are generally referred to as TGMR read sensors. The layers within a TGMR read sensor include an ultra-thin tunnel barrier layer positioned between a ferromagnetic pinned layer and a ferromagnetic free layer. As with SV read sensors, a magnetization of the free layer is allowed to rotate with respect to the pinned layer.
The transducing head also includes a magnetic writer, the writer including a writer core. The writer core generally includes an upper pole and a lower pole. A back via connects the upper and lower poles at a location spaced from the ABS. Disposed between the upper pole and lower pole is a writer coil, the writer coil including a number of turns of a conductor. A current passed through the writer coil creates a magnetic field. Modulation of the magnetic field generated by current in the writer coil allows the writer to record data by inducing a magnetization pattern on the magnetic disc.
A transducing head with a CPP reader typically uses the upper and lower shields to bring the sense current into and out of the reader. Because the MR read sensor is highly sensitive to noise, the upper and lower shields are isolated from noise sources. Noise generally refers to any undesired charges or currents introduced to the transducing head. The writer core is electrically isolated from the MR read sensor because the writer core can generate noise. In some CPP head configurations such as those with TGMR heads, an insulator, for example alumina (Al2O3) or silicon-dioxide (SiO2), generally insulates the upper shield of the reader from the lower pole of the writer. In such designs, there is a danger of discharge across the insulator between the writer and the reader. Such a discharge to the highly sensitive MR read sensor poses a risk of thermal breakdown of the MR read sensor, causing performance degradation.
The transducing head is supported above the disc by a slider, the slider comprising a substrate. Typically, the reader is placed upon the substrate. The writer is typically placed adjacent the reader. An insulating basecoat is typically disposed between the reader and the substrate.
Bleeder resistors are well-known as a means of electrically connecting the reader and the substrate across the basecoat to limit damage due to electrostatic discharge (ESD) and/or electrical overstress (EOS). ESD generally describes actual discharges, while EOS describes a condition where the circuitry is exposed to voltages or currents that are higher than under normal operating conditions, such as may be experienced during electrical testing; however, because ESD and EOS refer to functionally related conditions, use of either term is hereafter intended to contain the other term. ESD typically arises from triboelectric charging or induction. The reader bleeder resistor generally is a thin film resistor, having a resistance on the order of 1-3 Mega-ohms (MΩ), connected by a pair of vias to the reader and to the substrate. The reader bleeder resistor grounds the reader, typically grounding the reader to the electrically conductive substrate, to minimize or prevent ESD or EOS damage to the highly sensitive MR read sensor. The substrate is connected to the suspension assembly, which positions the transducing head relative to the disc. The suspension assembly is connected to the substrate with a conductive adhesive.
Reader bleeder resistors function to reduce accumulation of electrostatic charges on the reader, and thus limit damaging ESD events. A resistance of the reader bleeder resistor is selected to provide a path so that electrostatic charges on the highly sensitive reader are dissipated through the reader bleeder resistor to the substrate.
These reader bleeder resistors primarily protect the reader during fabrication of the HDD. However, because MR read sensors are highly sensitive to noise, reader bleeder resistors are constrained to high resistances to avoid introducing noise from the substrate to the reader. High resistances greatly reduce the likelihood of noise traveling through the reader bleeder resistor.
CPP heads use top and bottom magnetic shields to bring current into and out of the reader. The shields carry the sensor output to the preamp, and must therefore be isolated from noise sources, including the magnetic core of the writer. The preamp provides some control over voltages in the top and bottom shields. In traditional CIP heads, the shields and writer core are typically shorted together through a shared pole. However, some CIP configurations have the shields and writer core electrically isolated. In many CPP heads, such as TGMR heads, the writer core is electrically isolated. Isolation of the writer core allows the writer core voltage to “float”, meaning the voltage of the writer core can fluctuate independent of other elements in the transducing head. The floating voltage of the writer core can reach voltages of one volt or more due to capacitive coupling to the writer coil and also due to tribocharging with the disc.
A number of factors may generate ESD or EOS during fabrication and/or operation of the HDD. During operation, longitudinal and perpendicular writers may exhibit a writer core voltage of one volt or more from tribocharging of the pole tip features during ABS contact with particles or with the magnetic disc. Additionally, during fabrication, charged objects may contact the transducing head causing ESD or EOS.
Both the reader and writer are sensitive to discharges. Discharges generate heat, which leads to a risk of core-to-coil and coil-to-reader thermal breakdown. Even writer core voltages of less than one volt create damaging discharges between the transducing head and the magnetic disc. Discharges also lead to damaged discs, and material from the disc can be transferred to a surface of the transducing head. Such material transfer generates defects on the disc, creates deposits on the surface of the transducing head that induce further contact between the head and the disc, and also affects the fly characteristics of the slider.
In addition, many designs, such as those utilizing TGMR heads, have a separating oxide that insulates the upper shield of the reader from the lower pole of the writer. In such designs, there is a danger of discharge across the separating oxide between the writer and the reader. Such a discharge to the highly sensitive MR read sensor poses a risk of damages to the MR read sensor, causing performance degradation.
Thus, a transducing head is needed to improve drive reliability by grounding the writer to control ESD between the transducing head and the disc, both in frequency and magnitude.